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      Class 12 CHEMISTRY – JEE

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      • Chemistry
      • Class 12 CHEMISTRY – JEE
      CoursesClass 12ChemistryClass 12 CHEMISTRY – JEE
      • 1. Solid State
        11
        • Lecture1.1
          Crystalline & Amorphous Solid 50 min
        • Lecture1.2
          Law of Crystallography 01 hour
        • Lecture1.3
          Bravius lattice & Important Terms of solid state 48 min
        • Lecture1.4
          Type of Cubic crystal & Closest packed St. 01 hour
        • Lecture1.5
          Tetrahedral & Octahedral Void 38 min
        • Lecture1.6
          Type of Voids & Radius Ratio 44 min
        • Lecture1.7
          Type of ionic solid 59 min
        • Lecture1.8
          Defect in Solid 48 min
        • Lecture1.9
          Metallic Bonding 52 min
        • Lecture1.10
          Chapter Notes – Solid State
        • Lecture1.11
          NCERT Solutions – Solid State
      • 2. Solution and its C.P
        9
        • Lecture2.1
          Condition of solution formation, TD of Solution, Factors affecting solubility-Henary’s Law 55 min
        • Lecture2.2
          Colligative Properties, Raoult’s Law 49 min
        • Lecture2.3
          Relative lowering of V.P. & Problems 45 min
        • Lecture2.4
          Non ideal solution, Azeotropic Solution 46 min
        • Lecture2.5
          Elevation in B.P., Depression in F.P. 47 min
        • Lecture2.6
          Osmotic Pressure, Abnormal C.P. & Van’t Hoff Factor 59 min
        • Lecture2.7
          Solution – Ostwald Walker Exp. 13 min
        • Lecture2.8
          Chapter Notes – Solution and its C.P
        • Lecture2.9
          NCERT Solutions – Solution and its C.P
      • 3. Chemical Kinetics
        10
        • Lecture3.1
          Rate of reaction 37 min
        • Lecture3.2
          Differential Rate Law 38 min
        • Lecture3.3
          Integrated Rate Law 56 min
        • Lecture3.4
          Integrated Rate problems 53 min
        • Lecture3.5
          Pseudo order Reaction 40 min
        • Lecture3.6
          Reaction Mechanism 47 min
        • Lecture3.7
          Collision Model 34 min
        • Lecture3.8
          Arhenius Equation 34 min
        • Lecture3.9
          Chapter Notes – Chemical Kinetics
        • Lecture3.10
          NCERT Solutions – Chemical Kinetics
      • 4. Electrochemistry
        13
        • Lecture4.1
          Introduction & Galvanic cell 32 min
        • Lecture4.2
          Cell Notation & Cell Reaction 35 min
        • Lecture4.3
          Electrode & Cell Potential 38 min
        • Lecture4.4
          Electrochemical series 39 min
        • Lecture4.5
          The Nernst Equation 39 min
        • Lecture4.6
          Concentration cell, Battery, Corrosion 52 min
        • Lecture4.7
          Electrolysis 20 min
        • Lecture4.8
          Faraday Law 45 min
        • Lecture4.9
          Resistance & Conductance 40 min
        • Lecture4.10
          Molar & Eq. Conductance, Kohlraush’s Law 29 min
        • Lecture4.11
          Problems on Resistance & Conductance 23 min
        • Lecture4.12
          Chapter Notes – Electrochemistry
        • Lecture4.13
          NCERT Solutions – Electrochemistry
      • 5. Surface Chemistry
        11
        • Lecture5.1
          Introduction & Surface tension & surface energy 33 min
        • Lecture5.2
          Adsorption 47 min
        • Lecture5.3
          Factors affecting Adsorption 39 min
        • Lecture5.4
          Catalysis 34 min
        • Lecture5.5
          Type of Catalysis & Enzyme Catalysis 41 min
        • Lecture5.6
          Colloidal Solution 57 min
        • Lecture5.7
          Type of Colloidal Solution 43 min
        • Lecture5.8
          Properties of Colloidal Solution 50 min
        • Lecture5.9
          Protective Colloids 58 min
        • Lecture5.10
          Chapter Notes – Surface Chemistry
        • Lecture5.11
          NCERT Solutions – Surface Chemistry
      • 6. Alcohol & Ether
        8
        • Lecture6.1
          Preparation 35 min
        • Lecture6.2
          Physical Properties & Oxidation Of Alcohol 29 min
        • Lecture6.3
          Hydrates, Acetal, Ketal 38 min
        • Lecture6.4
          Tests Of Alcohol 47 min
        • Lecture6.5
          Ether Preparation & Its Properties 33 min
        • Lecture6.6
          Thiol & Thioether 16 min
        • Lecture6.7
          Chapter Notes – Alcohol & Ether
        • Lecture6.8
          NCERT Solutions – Alcohol & Ether
      • 7. Aldehyde & Ketone
        10
        • Lecture7.1
          Preparation 33 min
        • Lecture7.2
          Physical Properties, Beckmann Rearrangement, Witting Reaction 46 min
        • Lecture7.3
          Schmidt Reaction, Bayer Villegar Oxidation 22 min
        • Lecture7.4
          Aldol Condensation Reaction 40 min
        • Lecture7.5
          Cannizzaro Reaction 32 min
        • Lecture7.6
          Acyloin, Benzoin, Clasien, Perkin Condensation 28 min
        • Lecture7.7
          Reformasky Reaction, Tischenko Reaction 20 min
        • Lecture7.8
          Tests-8 40 min
        • Lecture7.9
          Chapter Notes – Aldehyde & Ketone
        • Lecture7.10
          NCERT Solutions – Aldehyde & Ketone
      • 8. Acid & derivatives
        4
        • Lecture8.1
          Preparation 31 min
        • Lecture8.2
          Chemical Reactions Of Acids 31 min
        • Lecture8.3
          Arndt Eistert, Curtius, Hvz, Hoffmann Reaction 19 min
        • Lecture8.4
          Acid Derivatives 38 min
      • 9. Nitrogen containing compounds
        4
        • Lecture9.1
          Alkyl Nitrites, Nitro Alkane 27 min
        • Lecture9.2
          Alkane Nitrile & Isonitrile 20 min
        • Lecture9.3
          Amine Preparation 24 min
        • Lecture9.4
          Properties Of Amines 13 min
      • 10. Aromatic Compounds
        7
        • Lecture10.1
          Benzene 41 min
        • Lecture10.2
          Aromatic Hydrocarbon 29 min
        • Lecture10.3
          Aryl Halides 18 min
        • Lecture10.4
          Phenol 40 min
        • Lecture10.5
          Aromatic Aldehyde 39 min
        • Lecture10.6
          Aniline 32 min
        • Lecture10.7
          Phenyl Diazonium Salts 37 min
      • 11. Biomolecules
        14
        • Lecture11.1
          Introduction & Types Of Carbohydrates 47 min
        • Lecture11.2
          D-glucose & D-fructose 50 min
        • Lecture11.3
          Reactions Of D-glucose & D-fructose 32 min
        • Lecture11.4
          Reactions Of D-glucose & D-fructose 23 min
        • Lecture11.5
          Sucrose, Maltose, Lactose 31 min
        • Lecture11.6
          Starch, Cellulose, Glycogen 27 min
        • Lecture11.7
          Reducing Sugar, Mutarotation, Osazone Formation 40 min
        • Lecture11.8
          Problems On Carbohydrates 41 min
        • Lecture11.9
          Amino Acids 48 min
        • Lecture11.10
          Peptides 47 min
        • Lecture11.11
          Proteins 18 min
        • Lecture11.12
          Enzyme & Vitamins 30 min
        • Lecture11.13
          Nucleic Acid 36 min
        • Lecture11.14
          Chapter Notes – Biomolecules
      • 12. Polymer Chemistry
        6
        • Lecture12.1
          Polymerisation Addition Reaction 32 min
        • Lecture12.2
          Coordination Addition, Condensation Reaction 24 min
        • Lecture12.3
          Division Of Polymer 41 min
        • Lecture12.4
          Examples Of Polymer 31 min
        • Lecture12.5
          Examples Of Polymer 31 min
        • Lecture12.6
          Chapter Notes – Polymer Chemistry
      • 13. Practical Organic Chemistry
        4
        • Lecture13.1
          Poc Qualitative Analysis 23 min
        • Lecture13.2
          Poc Qualitative Analysis 20 min
        • Lecture13.3
          Poc Quantitative Analysis 29 min
        • Lecture13.4
          Poc Quantitative Analysis 20 min
      • 14. P block elements II
        13
        • Lecture14.1
          VA – Elemental Properties of N family 51 min
        • Lecture14.2
          VA – Compounds of N family 43 min
        • Lecture14.3
          VA – N & Its compounds 45 min
        • Lecture14.4
          VA – Oxides & Oxyacids of Nitrogen 55 min
        • Lecture14.5
          VA – P & its compounds 31 min
        • Lecture14.6
          VA – Oxides & Oxyacids of P 31 min
        • Lecture14.7
          VIA 1 – Elemental Properties of O-Family 36 min
        • Lecture14.8
          VIA 2 – compounds of VIA elements 41 min
        • Lecture14.9
          VIA 3 – Oxygen & Ozone 47 min
        • Lecture14.10
          VIA 4 – Sulphur & oxides of Sulphur 37 min
        • Lecture14.11
          VIA 5 – Sulphuric Acid 25 min
        • Lecture14.12
          Chapter Notes – P block elements
        • Lecture14.13
          NCERT Solutions – P block elements
      • 15. P block elements III
        5
        • Lecture15.1
          VIIA 1 – elemental properties of Halogen 40 min
        • Lecture15.2
          VIIA 2 – Compounds of Halogen 49 min
        • Lecture15.3
          VIIA 3 – Chlorine & its Compounds 41 min
        • Lecture15.4
          VIIIA 1 – Properties of Noble Gas 34 min
        • Lecture15.5
          VIIIA 2 – Compounds of Noble Gas 34 min
      • 16. D block metals
        8
        • Lecture16.1
          D block – Elemental Properties 55 min
        • Lecture16.2
          Elemental Properties 01 hour
        • Lecture16.3
          Elemental Properties 53 min
        • Lecture16.4
          KMnO4 & K2Cr2O7 47 min
        • Lecture16.5
          Problems 40 min
        • Lecture16.6
          Problems 20 min
        • Lecture16.7
          Chapter Notes – The d-and f-Block Elements
        • Lecture16.8
          NCERT Solutions – The d-and f-Block Elements
      • 17. F block metals
        3
        • Lecture17.1
          Lanthanoids 52 min
        • Lecture17.2
          Actinoids 48 min
        • Lecture17.3
          Problems 42 min
      • 18. Co-ordination compounds
        17
        • Lecture18.1
          Introduction of Complex Compound, Ligands 42 min
        • Lecture18.2
          Classification of Ligands, Denticity 35 min
        • Lecture18.3
          Nomenclature of Complex Compounds 46 min
        • Lecture18.4
          Nomenclature of Complex Compounds 2 40 min
        • Lecture18.5
          Bonding in Complex Compound, Primary & Secondary Valency 44 min
        • Lecture18.6
          Concept of EAN 29 min
        • Lecture18.7
          VBT in Complex Compounds 58 min
        • Lecture18.8
          Examples on VBT in complex compounds 31 min
        • Lecture18.9
          CFT in Complex Compounds 43 min
        • Lecture18.10
          CFT for Octahedral & Tetrahedral Complex 35 min
        • Lecture18.11
          Colour & Stability of Complex Compounds 28 min
        • Lecture18.12
          Structural Isomerism in Complex Compounds 49 min
        • Lecture18.13
          Geometrical Isomerism in Complex Compounds 43 min
        • Lecture18.14
          Optical Isomerism in Complex Compounds, use of Complex 01 hour
        • Lecture18.15
          Organometallic Compounds 29 min
        • Lecture18.16
          Chapter Notes – Co-ordination compounds
        • Lecture18.17
          NCERT Solutions – Co-ordination compounds
      • 19. Environmental Chemistry
        4
        • Lecture19.1
          Introduction & Air Pollution 35 min
        • Lecture19.2
          Air Pollution 20 min
        • Lecture19.3
          Water Pollution 23 min
        • Lecture19.4
          Soil Pollution, Prevention of Pollution 16 min

        NCERT Solutions – Surface Chemistry

        5.1. Distinguish between the meaning of the terms adsorption and absorption.

        Give one example of each.

        Answer

        Adsorption is a surface phenomenon of accumulation of molecules of a substance at the surface rather than in the bulk of a solid or liquid. The substance that gets adsorbed is called the ‘adsorbate’ and the substance on whose surface the adsorption takes place is called the ‘adsorbent’. Here, the concentration of the adsorbate on the surface of the adsorbent increases. In adsorption, the substance gets concentrated at the surface only. It does not penetrate through the surface to the bulk of the solid or liquid. For example, when we dip a chalk stick into an ink solution, only its surface becomes coloured. If we break the chalk stick, it will be found to be white from inside.

        On the other hand, the process of absorption is a bulk phenomenon. In absorption, the substance gets uniformly distributed throughout the bulk of the solid or liquid.

        5.2. What is the difference between physisorption and chemisorption?

        Answer

        Physisorption

        Chemisorption

        1.

        In this type of adsorption, the adsorbate is attached to the surface of the adsorbent with weak van der Waal’s forces of attraction.

        In this type of adsorption, strong chemical bonds are formed between the adsorbate and the surface of the adsorbent.

        2.

        No new compound is formed in the process.

        New compounds are formed at the surface of the adsorbent.

        3.

        It is generally found to be reversible in nature.

        It is usually irreversible in nature.

        4.

        Enthalpy of adsorption is low as weak van der Waal’s forces of attraction are involved. The values lie in the range of 20-40 kJ mol−1.

        Enthalpy of adsorption is high as chemical bonds are formed. The values lie in the range of 40-400 kJ mol−1.

        5.

        It is favoured by low temperature conditions.

        It is favoured by high temperature conditions.

        6.

        It is an example of multi-layer adsorption

        It is an example of mono-layer adsorption.

        5.3. Give reason why a finely divided substance is more effective as an adsorbent.

        Answer

        Adsorption is a surface phenomenon. Therefore, adsorption is directly proportional to the surface area. A finely divided substance has a large surface area. Both physisorption and chemisorption increase with an increase in the surface area. Hence, a finely divided substance behaves as a good adsorbent.

        5.4. What are the factors which influence the adsorption of a gas on a solid?

        Answer

        There are various factors that affect the rate of adsorption of a gas on a solid surface.

        (1) Nature of the gas:

        Easily liquefiable gases such as NH3, HCl etc. are adsorbed to a great extent in comparison to gases such as H2, O2 etc. This is because Van der Waal’s forces are stronger in easily liquefiable gases.

        (2) Surface area of the solid

        The greater the surface area of the adsorbent, the greater is the adsorption of a gas on the solid surface.

        (3) Effect of pressure

        Adsorption is a reversible process and is accompanied by a decrease in pressure. Therefore, adsorption increases with an increase in pressure.

        (4) Effect of temperature

        Adsorption is an exothermic process. Thus, in accordance with Le-Chatelier’s principle, the magnitude of adsorption decreases with an increase in temperature.

        5.5. What is an adsorption isotherm? Describe Freundlich adsorption isotherm.

        Answer

        The plot between the extent of adsorption  against the pressure of gas (P) at constant temperature (T) is called the adsorption isotherm.

        Freundlich adsorption isotherm:

        Freundlich adsorption isotherm gives an empirical relationship between the quantity of gas adsorbed by the unit mass of solid adsorbent and pressure at a specific temperature.

        From the given plot it is clear that at pressure PS, reaches the maximum valve. Ps is called the saturation pressure. Three cases arise from the graph now.

        Case I- At low pressure:

        The plot is straight and sloping, indicating that the pressure in directly proportional to i.e., 

        Case II- At high pressure:

        When pressure exceeds the saturated pressure, becomes independent of P values.

        Case III- At intermediate pressure:

        At intermediate pressure,  depends on P raised to the powers between 0 and 1. This relationship is known as the Freundlich adsorption isotherm.

        On plotting the graph between log and log P, a straight line is obtained with the slope equal to and the intercept equal to log k.

        5.6. What do you understand by activation of adsorbent? How is it achieved?

        Answer

        By activating an adsorbent, we tend to increase the adsorbing power of the adsorbent. Some ways to activate an adsorbent are:

        (i) By increasing the surface area of the adsorbent. This can be done by breaking it into smaller pieces or powdering it.

        (ii) Some specific treatments can also lead to the activation of the adsorbent. For example, wood charcoal is activated by heating it between 650 K and 1330 K in vacuum or air. It expels all the gases absorbed or adsorbed and thus, creates a space for adsorption of gases.

        5.7. What role does adsorption play in heterogeneous catalysis?

        Answer

        Heterogeneous catalysis:

        A catalytic process in which the catalyst and the reactants are present in different phases is known as a heterogeneous catalysis. This heterogeneous catalytic action can be explained in terms of the adsorption theory. The mechanism of catalysis involves the following steps:

        (i) Adsorption of reactant molecules on the catalyst surface.

        (ii) Occurrence of a chemical reaction through the formation of an intermediate.

        (iii) De-sorption of products from the catalyst surface

        (iv) Diffusion of products away from the catalyst surface.

        In this process, the reactants are usually present in the gaseous state and the catalyst is present in the solid state. Gaseous molecules are then adsorbed on the surface of the catalyst. As the concentration of reactants on the surface of the catalyst increases, the rate of reaction also increases. In such reactions, the products have very less affinity for the catalyst and are quickly desorbed, thereby making the surface free for other reactants.

        5.8. Why is adsorption always exothermic?

        Answer

        Adsorption is always exothermic. This statement can be explained in two ways.

        (i) Adsorption leads to a decrease in the residual forces on the surface of the adsorbent. This causes a decrease in the surface energy of the adsorbent. Therefore, adsorption is always exothermic.

        (ii) ΔH of adsorption is always negative. When a gas is adsorbed on a solid surface, its movement is restricted leading to a decrease in the entropy of the gas i.e., ΔS is negative. Now for a process to be spontaneous, ΔG should be negative.

        ΔG = ΔH − TΔS

        Since ΔS is negative, ΔH has to be negative to make ΔG negative. Hence, adsorption is always exothermic.

        5.9. How are the colloidal solutions classified on the basis of physical states of the dispersed phase and dispersion medium?

        Answer

        One criterion for classifying colloids is the physical state of the dispersed phase and dispersion medium. Depending upon the type of the dispersed phase and dispersion medium (solid, liquid, or gas), there can be eight types of colloidal systems.

         

        Dispersed phase

        Dispersion medium

        Type of colloid

        Example

        1.

        Solid

        Solid

        Solid Sol

        Gemstone

        2.

        Solid

        Liquid

        Sol

        Paint

        3.

        Solid

        Gas

        Aerosol

        Smoke

        4.

        Liquid

        Solid

        Gel

        Cheese

        5.

        Liquid

        Liquid

        Emulsion

        Milk

        6.

        Liquid

        Gas

        Aerosol

        Fog

        7.

        Gas

        Solid

        Solid foam

        Pumice stone

        8.

        Gas

        Liquid

        Foam

        Froth

         

        5.10. Discuss the effect of pressure and temperature on the adsorption of gases on solids.

        Answer

        Effect of pressure

        Adsorption is a reversible process and is accompanied by a decrease in pressure. Therefore, adsorption increases with an increase in pressure.

        Effect of temperature

        Adsorption is an exothermic process. Thus, in accordance with Le-Chatelier’s principle, the magnitude of adsorption decreases with an increase in temperature.

        5.10.What are lyophilic and lyophobic sols? Give one example of each type. Why are hydrophobic sols easily coagulated?

        Answer

        (i) Lyophilic sols:

        Colloidal sols that are formed by mixing substances such as gum, gelatin, starch, etc. with a suitable liquid (dispersion medium) are called lyophilic sols. These sols are reversible in nature i.e., if two constituents of the sol are separated by any means (such as evaporation), then the sol can be prepared again by simply mixing the dispersion medium with the dispersion phase and shaking the mixture.

        (ii) Lyophobic sols:

        When substances such as metals and their sulphides etc. are mixed with the dispersion medium, they do not form colloidal sols. Their colloidal sols can be prepared only by special methods. Such sols are called lyophobic sols. These sols are irreversible in nature. For example: sols of metals.

        Now, the stability of hydrophilic sols depends on two things- the presence of a charge and the salvation of colloidal particles. On the other hand, the stability of hydrophobic sols is only because of the presence of a charge. Therefore, the latter are much less stable than the former. If the charge of hydrophobic sols is removed (by addition of electrolytes), then the particles present in them come closer and form aggregates, leading to precipitation.

        5.12. What is the difference between multimolecular and macromolecular colloids? Give one example of each. How are associated colloids different from these two types of colloids?

        Answer

        (i) In multi-molecular colloids, the colloidal particles are an aggregate of atoms or small molecules with a diameter of less than 1 nm. The molecules in the aggregate are held together by van der Waal’s forces of attraction. Examples of such colloids include gold sol and sulphur sol.

        (ii) In macro-molecular colloids, the colloidal particles are large molecules having colloidal dimensions. These particles have a high molecular mass. When these particles are dissolved in a liquid, sol is obtained. For example: starch, nylon, cellulose, etc.

        (iii) Certain substances tend to behave like normal electrolytes at lower concentrations. However, at higher concentrations, these substances behave as colloidal solutions due to the formation of aggregated particles. Such colloids are called aggregated colloids.

        5.13. What are enzymes? Write in brief the mechanism of enzyme catalysis.

        Answer

        Enzymes are basically protein molecules of high molecular masses. These form colloidal solutions when dissolved in water. These are complex, nitrogenous organic compounds produced by living plants and animals. Enzymes are also called ‘biochemical catalysts’.

        Mechanism of enzyme catalysis:

        On the surface of the enzymes, various cavities are present with characteristic shapes. These cavities possess active groups such as −NH2, −COOH, etc. The reactant molecules having a complementary shape fit into the cavities just like a key fits into a lock. This leads to the formation of an activated complex. This complex then decomposes to give the product.

        Hence,

        Step 1: E + S → ES+

        (Activated complex)

        Step 2: ES+ → E + P

        5.14. How are colloids classified on the basis of

        (i) Physical states of components

        (ii) Nature of dispersion medium and

        (iii) Interaction between dispersed phase and dispersion medium?

        Answer

        Colloids can be classified on various bases:

        (i) On the basis of the physical state of the components (by components we mean the dispersed phase and dispersion medium). Depending on whether the components are solids, liquids, or gases, we can have eight types of colloids.

        (ii) On the basis of the dispersion medium, sols can be divided as:

        Dispersion medium

        Name of sol

        Water

        Aquasol or hydrosol

        Alcohol

        Alcosol

        Benzene

        Benzosol

        Gases

        Aerosol

        (iii) On the basis of the nature of the interaction between the dispersed phase and dispersion medium, the colloids can be classified as lyophilic (solvent attracting) and lyophobic (solvent repelling).

        5.15. Explain what is observed

        (i) When a beam of light is passed through a colloidal sol.

        (ii) An electrolyte, NaCl is added to hydrated ferric oxide sol.

        (iii) Electric current is passed through a colloidal sol?

        Answer

        (i) When a beam of light is passed through a colloidal solution, then scattering of light is observed. This is known as the Tyndall effect. This scattering of light illuminates the path of the beam in the colloidal solution.

        (ii) When NaCl is added to ferric oxide sol, it dissociates to give Na+ and Cl–­ ions. Particles of ferric oxide sol are positively charged. Thus, they get coagulated in the presence of negatively charged Cl– ions.

        (iii) The colloidal particles are charged and carry either a positive or negative charge. The dispersion medium carries an equal and opposite charge. This makes the whole system neutral. Under the influence of an electric current, the colloidal particles move towards the oppositely charged electrode. When they come in contact with the electrode, they lose their charge and coagulate.

        5.16. What are emulsions? What are their different types? Give example of each type.

        Answer

        The colloidal solution in which both the dispersed phase and dispersion medium are liquids is called an emulsion.

        There are two types of emulsions:

        (a) Oil in water type:

        Here, oil is the dispersed phase while water is the dispersion medium. For example: milk, vanishing cream, etc.

        (b) Water in oil type:

        Here, water is the dispersed phase while oil is the dispersion medium. For example: cold cream, butter, etc.

        5.17. What is demulsification? Name two demulsifiers.

        Answer

        The process of decomposition of an emulsion into its constituent liquids is called demulsification. Examples of demulsifiers are surfactants, ethylene oxide, etc.

        5.18. Action of soap is due to emulsification and micelle formation. Comment.

        Answer

        The cleansing action of soap is due to emulsification and micelle formation. Soaps are basically sodium and potassium salts of long chain fatty acids, R-COO–Na+. The end of the molecule to which the sodium is attached is polar in nature, while the alkyl-end is non-polar. Thus, a soap molecule contains a hydrophilic (polar) and a hydrophobic (non-polar) part.

        When soap is added to water containing dirt, the soap molecules surround the dirt particles in such a manner that their hydrophobic parts get attached to the dirt molecule and the hydrophilic parts point away from the dirt molecule. This is known as micelle formation. Thus, we can say that the polar group dissolves in water while the non-polar group dissolves in the dirt particle. Now, as these micelles are negatively charged, they do not coalesce and a stable emulsion is formed.

        5.19. Give four examples of heterogeneous catalysis.

        Answer

        (i) Oxidation of sulphur dioxide to form sulphur trioxide. In this reaction, Pt acts as a catalyst.

        (ii) Formation of ammonia by the combination of dinitrogen and dihydrogen in the presence of finely divided iron.

        This process is called the Haber’s process.

        (iii) Oswald’s process: Oxidation of ammonia to nitric oxide in the presence of platinum.

        (iv) Hydrogenation of vegetable oils in the presence of Ni.

        5.20. What do you mean by activity and selectivity of catalysts?

        Answer

        (a) Activity of a catalyst:

        The activity of a catalyst is its ability to increase the rate of a particular reaction. Chemisorption is the main factor in deciding the activity of a catalyst. The adsorption of reactants on the catalyst surface should be neither too strong nor too weak. It should just be strong enough to make the catalyst active.

        (b) Selectivity of the catalyst:

        The ability of the catalyst to direct a reaction to yield a particular product is referred to as the selectivity of the catalyst. For example, by using different catalysts, we can get different products for the reaction between H2 and CO.

        (i) 

        (ii) 

        (iii) 

        5.21. Describe some features of catalysis by zeolites.

        Answer

        Zeolites are alumino-silicates that are micro-porous in nature. Zeolites have a honeycomb-like structure, which makes them shape-selective catalysts. They have an extended 3D-network of silicates in which some silicon atoms are replaced by aluminium atoms, giving them an Al−O−Si framework. The reactions taking place in zeolites are very sensitive to the pores and cavity size of the zeolites. Zeolites are commonly used in the petrochemical industry.

        5.22. What is shape selective catalysis?

        Answer

        A catalytic reaction which depends upon the pore structure of the catalyst and on the size of the reactant and the product molecules is called shape-selective catalysis. For example, catalysis by zeolites is a shape-selective catalysis. The pore size present in the zeolites ranges from 260-740 pm. Thus, molecules having a pore size more than this cannot enter the zeolite and undergo the reaction.

        5.23. Explain the following terms:

        (i) Electrophoresis (ii) Coagulation

        (iii) Dialysis (iv) Tyndall effect.

        Answer

        (i) Electrophoresis:

        The movement of colloidal particles under the influence of an applied electric field is known as electrophoresis. Positively charged particles move to the cathode, while negatively charged particles move towards the anode. As the particles reach oppositely charged electrodes, they become neutral and get coagulated.

        (ii) Coagulation:

        The process of settling down of colloidal particles i.e., conversion of a colloid into a precipitate is called coagulation.

        (iii) Dialysis

        The process of removing a dissolved substance from a colloidal solution by the means of diffusion through a membrane is known as dialysis. This process is based on the principle that ions and small molecules can pass through animal membranes unlike colloidal particles.

        (iv) Tyndall effect:

        When a beam of light is allowed to pass through a colloidal solution, it becomes visible like a column of light. This is known as the Tyndall effect. This phenomenon takes place as particles of colloidal dimensions scatter light in all directions.

        5.24. Give four uses of emulsions.

        Answer

        Four uses of emulsions:

        (i) Cleansing action of soaps is based on the formation of emulsions.

        (ii) Digestion of fats in intestines takes place by the process of emulsification.

        (iii) Antiseptics and disinfectants when added to water form emulsions.

        (iv) The process of emulsification is used to make medicines.

        5.25. What are micelles? Give an example of a micellers system.

        Answer

        Micelle formation is done by substances such as soaps and detergents when dissolved in water. The molecules of such substances contain a hydrophobic and a hydrophilic part. When present in water, these substances arrange themselves in spherical structures in such a manner that their hydrophobic parts are present towards the centre, while the hydrophilic parts are pointing towards the outside (as shown in the given figure). This is known as micelle formation.

         

        5.26. Explain the terms with suitable examples:

        (i) Alcosol (ii) Aerosol (iii) Hydrosol

        Answer

        (i) Alcosol:

        A colloidal solution having alcohol as the dispersion medium and a solid substance as the dispersed phase is called an alcosol.

        For example: colloidal sol of cellulose nitrate in ethyl alcohol is an alcosol.

        (ii) Aerosol:

        A colloidal solution having a gas as the dispersion medium and a solid as the dispersed phase is called an aerosol.

        For example: fog

        (iii) Hydrosol

        A colloidal solution having water as the dispersion medium and a solid as the dispersed phase is called a hydrosol.

        For example: starch sol or gold sol

        5.27. Comment on the statement that “colloid is not a substance but a state of substance”.

        Answer

        Common salt (a typical crystalloid in an aqueous medium) behaves as a colloid in a benzene medium. Hence, we can say that a colloidal substance does not represent a separate class of substances. When the size of the solute particle lies between 1 nm and 1000 nm, it behaves as a colloid.

        Hence, we can say that colloid is not a substance but a state of the substance which is dependent on the size of the particle. A colloidal state is intermediate between a true solution and a suspension.

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